1. Field of the Invention
The invention relates to a process for producing a premium, synthetic lubricant base stock produced from waxy, Fischer-Tropsch synthesized hydrocarbons. More particularly the invention relates to an isoparaffinic lubricant base stock produced by hydroisomerizing a waxy, paraffinic Fischer-Tropsch synthesized hydrocarbon fraction and catalytically dewaxing the hydroisomerate with a Pt/H-mordenite dewaxing catalyst.
2. Background of the Invention
Current trends in the design of automotive engines require higher quality crankcase and transmission lubricating oils having a high viscosity index (VI) and low pour point. While high VI's have typically been achieved with the use of VI improvers as additives to the oil, additives are expensive and tend to undergo degradation from the high engine temperatures and shear rates. Processes for preparing lubricating oils of low pour point from petroleum derived feeds typically include atmospheric and/or vacuum distilling a crude oil to recover fractions boiling in the lubricating oil range, solvent extracting the lubricating oil fractions to remove aromatics and form a raffinate, hydrotreating the raffinate to remove heteroatom compounds and aromatics, followed by either solvent or catalytically dewaxing the hydrotreated raffinate to reduce the pour point of the oil. More recently it has been found that good quality lubricating oils can be formed from hydrotreated slack wax and Fischer-Tropsch wax.
Fischer-Tropsch wax is a term used to describe waxy hydrocarbons produced by a Fischer-Tropsch hydrocarbon synthesis processes, in which a synthesis gas feed comprising a mixture of H.sub.2 and CO reacts in the presence of a Fischer-Tropsch catalyst, under conditions effective to form hydrocarbons. U.S. Pat. No. 4,963,672 discloses a process for converting waxy Fischer-Tropsch hydrocarbons to a lubricant base stock having a high VI and a low pour point by sequentially hydrotreating, hydroisomerizing, and solvent dewaxing. A preferred embodiment comprises sequentially (i) severely hydrotreating the wax to remove impurities and partially convert the 1050.degree. F.+ wax, (ii) hydroisomerizing the hydrotreated wax with a noble metal on a fluorided alumina catalyst, (iii) hydrorefining the hydroisomerate, (iv) fractionating the hydroisomerate to recover a lube oil fraction, and (v) solvent dewaxing the lube oil fraction to produce the base stock. European patent publication EP 0 668 342 A1 suggests a processes for producing lubricating base oils by hydrogenating and then hydroisomerizing a waxy Fischer-Tropsch raffinate, followed by dewaxing. The hydrogenating is performed without cracking to lower the hydroisomerization temperature and increase the catalyst life, both of which those skilled in the art know are adversely effected by the presence of oxygenates and heteroatoms in the waxy feed. EP 0 776 959 A2 recites hydroconverting Fischer-Tropsch hydrocarbons having a narrow boiling range, fractionating the hydroconversion effluent into heavy and light fractions and then dewaxing the heavy fraction to form a lubricating base oil having a VI of at least 150.